Method and apparatus for recycling dryer stack gases



NOV. 10, 1970 c, wElMER ETAL 3,538,614

METHOD AND APPARATUS FOR RECYCLING DRYER STACK GASES Filed Sept. 9, 1968 INVENTCRS ERVIN C. WEIMER HAROLD W. SHIDELER STUART M. PORTER A ORNEYS United States Patent 3,538,614 METHOD AND APPARATUS FOR RECYCLING DRYER STACK GASES Ervin C. Weimer and Harold W. Shideler, Wheat Ridge, and Stuart M. Porter, Denver, Colo., assignors to The Stearns-Roger Corporation, Denver, Colo., a corporation of Colorado Filed Sept. 9, 1968, Ser. No. 758,412 Int. Cl. F26b 3/32 US. Cl. 34-28 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an improved method and apparatus for recycling dryer stack gases wherein said gases, along with the solids entrained therein, are drawn off tangentially from the product-recovery cyclones and returned to the system at a point within the combustion zone near the discharge end of the latter where these oxygen-lean recycled stack gases will not inhibit combustion in the furnace, yet will combine with the primary combustion gases to produce a pre-warmed gas mixture for drying the wet pulp in the dryer that will not support combustion therein and is essentially inert in the sense that it will not bring about oxidative degradation of the product. At the same time, the solids are being burned to eliminate them as atmospheric contaminants and their combustion heat is reclaimed to assist in the drying of the product. An essential feature of the system is that all components thereof, together with the stack gases flowing therethrough, be maintained at a temperature above the dew point of the latter so that no condensation can occur.

The conventional pulp-drying installations include a gas-fired furnace whose gaseous products of combustion are fed into a rotating drum-type dryer containing wet pulp. The dried product emerging from the discharge end of the dryer is either recovered directly in a suitable recovery vessel or else sucked up by an induced draft fan and fed to one or more cyclone-type recovery vessels where the dry product is drawn off the bottom while the gaseous elements are discharged through the top. In installations using both primary and secondary product-recovery systems, it is generally recognized that the secondary recovery units (the cyclones) operate more efiiciently when they have less product to handle but, even so, some solids still pass into the stack and are discharged into the atmosphere as pollutants. Thus, in both of these types of pulp-drying installations, it would be highly beneficial to be able to recycle the stack gases so as to eliminate or at least further reduce the solids entrained therein. One would naturally expect that a reduction in fuel costs would also result from such a stack gas recycling system.

Despite the obvious advantages attendant to recycling of the dryer stack gases back into the system, the prior art attempts along this line have proven unworkable and have been abandoned. One of the most serious problems encountered was that of the ducts becoming clogged with damp dust. Undoubtedly, other problems were encountered to which no solution was readily apparent and, therefore, further use of the recycling system became undesirable.

It has now been found in accordance with the teaching of the instant invention that a, dryer stack gas recycling ice.

' acting to inhibit combustion. Also, these same lean recycled gases can be combined with the gaseous products of primary combustion to produce a substantially inert mixture which will not support combustion in the dryer section or bring about detrimental oxidation of the product. By using the recycled stack gases to cool the hot, undiluted primary combustion gases, the conventional practice of using secondary air from the atmosphere as a coolant is eliminated. All components of the recycling system, along with the gases flowing therethrough, must, in accordance with the present invention, be maintained at a temperature somewhat higher than the dew point of these recycled gases; otherwise, the condensible fractions will condense out on the cold surfaces and entrap the entrained solids so as to eventually clog the ductwork.

While the amount of heat recovered by burning the entrained solids is not great, it, together with the heat saved by using the already warm stack gas as secondary air instead of air from the atmosphere, combine to bring about an overall fuel saving of somewhere between 5 and 8%.

In those installations which rely solely on the cyclones for recovery of the dried pulp, the amount of solids escaping with the stack gases is probably going to be greater due to reduced cyclone efficiency when required to handle the entire product output; therefore, the recycling system of the instant invention should prove even more advantageous than in those installations having both primary and secondary product recovery. In both types, however, the pollutants escaping into the atmosphere are going to be largely eliminated and their recovery is unimportant because tests have shown that these solids consist mostly of charred material having little, if any, value.

It is, therefore, the principal object of the present invention to provide a novel and improved method and apparatus for recycling dryer stack gases, along with the fine solids entrained therein.

A second objective is the provision of a dryer stack gas recycling method wherein the temperatures are maintained above the dew point of the gases so as to prevent condensation and the entrapment of solids in said condensate.

Another object of the invention herein disclosed and claimed is to provide a system for recirculating stack gases from a rotating drum dryer installation wherein the already warm recirculated gases are used as a coolant for the hot undiluted products of primary combustion in place of atmospheric air drawn in from the outside at colder ambient temperatures.

Still another objective of the aforementioned invention is to provide a recirculating system for dryer stack gases that largely eliminates the escape of solid pollutants into the atmosphere through the cyclone stacks.

An additional object is to provide a mixture of primary combustion gases and recycled stack gases for introduction into the dryer that is so depleted of its oxygen that it will not support combustion or bring about degrada tion of the product due to oxidation thereof.

A further object is to provide a stack gas recycling system that is readily incorporated into existing rotary-drum pulp-drying installations, one that results in a material reduction in fuel costs, and a unit of the type aforementioned that results in a better quality and more consistent product.

Other objects will be in part apparent and in part pointed out specifically hereinafter in connection with the detailed description of the single figure of the drawings that shows, somewhat schematically, a conventional rotary-drum-type pulp dryer installation equipped with the stack gas recycling system of the present invention, together with means for effecting automatic control thereof.

Reference numeral designates in a general way a representative pulp-drying installation of the type having a furnace 12 connected to receive gaseous fuel through fuel line 14 and discharge the hot gaseous products of combustion into rotary-drum dryer 16 that contains the pulp (not shown) which is to be dried. The dried product is discharged from the dryer, in the particular installation illustrated, into a primary product-recovery vessel 18 where it is collected from the bottom thereof. The moisture-laden gas from the dryer, carrying somewhere in the neighborhood of 10% of the dried product, is sucked from the discharge end of the dryer through damper 20 and delivered tangentially to the hollow cylindrical midsections 22 of cyclone separators 24 by induced draft fan 26. Most of the product remaining entrained in the moving gas stream is separated centrifugally in the cyclones 24 and is recoverd from the bottom thereof at the outlets of conical sections 28. Ordinarily, the gases, together with whatever solids remain suspended therein that have escaped separation in both the recovery vessel 18 and the cyclones 24, are discharged to the atmosphere through cyclone stacks 30.

It is to just such a pulp-drying installation as that which has been described above that the recycle system of the present invention is added. The furnace section 12 is, of course, stationary and it houses the combustion zone of the unit. The hollow cylindrical dryer drum 16 is mounted for slow rotational movement about its longitudinal axis upon trunnion blocks 32 which, in the particular form shown, lie spaced on opposite sides of the gear drive therefor which has been designated by numeral 34.

The intake end of the dryer drum telescopes over stationary tubular throat 36 at the discharge end of the furnace and rotates relative thereto. This tubular throat is provided with a wet pulp intake tube 38 that receives the wet pulp to be dried from some sort of transport mechanism (not shown) like, for example, a screw conveyor. The pulp, therefore, enters the system downstream of the combustion zone where, as will be shown presently, the hot undiluted products of primary combustion in the furnace have been cooled considerably by mixing same with cooler stack gases preparatory to delivering said mixture to the drying zone within the dryer.

At the discharge end of the dryer drum, a primary product recovery vessel 18 has been shown which, as aforesaid, separates somewhere around 90% of the dried product from the system. While such a primary product recovery step is desirable to reduce the cyclone load and thereby increase their efficiency, vessel 18 can be eliminated and the entire output fed directly to the cyclones. In either case, a small proportion of finely-divided solids, usually in the form of charred product which is not worth recovering, passes out the cyclone stacks with the moisture-laden waste gases.

Now, the present invention contemplates the addition of a dryer gas recycle system to the above-described conventional pulp-drying installation, said recycle system having been indicated in a general way on the drawing by reference numeral 40. Ducts 42 are connected tangentially into the stack of each cyclone 24 so as to draw off the stack gases spiralling circumferentially upward on the inside periphery thereof that contain substantially all of the left-over solids, that portion of the stack gas circulating nearer the center of the stack being substantially free of entrained dust. A second induced draft fan 44 connected into these stack ducts 42, sucks the stack gases and entrained solids from the cyclones 24 before they can enter the atmosphere and returns them to the furnace 12 through a return duct 46 at a point adjacent the downstream end of the combustion zone, but ahead of the drying zone and the point at which the wet pulp is introduced. Ducts 42 include dampers 48 that control the volume of gas recycled to the furnace.

It has been mentioned previously that the recycled gas and entrained solids are returned to the furnace adjacent the downstream end of the combustion zone. Since this is a critical factor in the proper operation of the recycle system, it would, perhaps, be wise to amplify some on this point.

To begin with, one of the main objectives of the recycle system is to eliminate the solid pollutants which would otherwise be discharged to the atmosphere. If, as aforementioned, we are going to get rid of these solids by incinerating them, then, obviously, they must be returned to the system at a point where this will occur; otherwise, they would merely continue to circulate through the system in ever-increasing quantities. Thus, it becomes imperative that the solids be returned to the system within the combustion zone where it is still hot enough to incinerate same. On the other hand, the stack gases carrying these solid pollutants are substantially devoid of oxygen, the oxygen content thereof running generally somewhere around /2 to 1%. If, therefore, this stack gas were returned to the combustion zone near the upstream end thereof where the fuel and oxygen-rich primary combustion air enters the system, its effect would be to retard combustion in the very area where excellent combustion is essential. Furthermore, the recycled stack gases would tend to cool the combustion zone if introduced near the lead or upstream end thereof and it is essential for proper combustion that the fuel flames remain quite hot. Accordingly, it is essential that the stack gases and retained solids be returned to the system far enough downstream in the combustion zone where they will not retard combustion or cool off said zone appreciably. On

the other hand, these recycled components must reenter the system where there is enough heat left to incinerate the solids and, as previously stated, these requirements are met by bringing the stack gases back into the system adjacent the downstream or discharge end of the combustion zone.

It so happens that returning the gases and entrained solids to the system at this point also has distinct advantages in terms of fuel savings. While the temperatures in the furnace should be quite hot to insure proper combustion, the gases used to dry the wet pulp must be considerably cooler for best results. Therefore, in the conventional rotating-drum-type dryer installations, secondary air from the atmosphere is mixed with the hot gaseous products of combustion from the furnace to cool off the latter preparatory to indroducing same into the drying zone. This air from the atmosphere is, of course, rich in oxygen when compared with the recycled stack gases, and it is also a great deal cooler.

Use of the recycled stack gases at a temperature of somewhere between approximately 240 F. and 290 F. as the secondary cooling air instead of air drawn from the atmosphere at, say, 70 F., brings about significant savings in fuel costs. Also, the combining of the hot gaseous primary combustion products with the oxygenlean recycled stack gases results in a mixture which is substantially inert in that it will not support combustion in the drying zone, nor will it contribute to an oxidative degradation of the pulp. To accomplish these ends, the recycled gases must enter the system ahead of the drying zone and ahead of the point where the wet pulp is introduced in order to cool ofi the primary products of combustion to a temperature of about 1200 F. before they enter the dryer or come into contact with the pulp.

Existing drum-type pulp-drying installations commonly provide for automatic control of the system by regulating the amount of fuel fed to the furnace with an automatic flow control valve 50 connected into fuel line 14. A tem perature responsive element 56 located in the primary product recovery vessel in position to measure the temperature of the stack gases leaving the dryer, sends information on the temperature at this point to a suitable control mechanism that has been designated schematically by box 54 and which functions to regulate the fuel flow so as to keep the exhaust gas at a predetermined temperature. Thus, an increase in wet feed material to the dryer tends to reduce the temperature of the exhaust gases, and the control as described, functions to open the fuel valve to satisfy the new conditions. A temperature element 52 housed inside the throat 36 sends information on the temperature at this point to the control mechanism 54. This signal will act to override the basic control of the fuel valve should there be a malfunction or if, for any other reason, the temperature in the throat exceeds a preselected value. In addition, the temperature of the moisture-laden exhaust gases ordinarily discharged through the stacks can be maintained at a level above the dew point until it reenters the combustion zone of the system so that no condensation of the condensible fractions contained therein will take place in the recirculation ductwork. Such temperature measurement and fuel flow control is desirable in existing pulp-drying installations, whether controlled manually or automatically, to maintain a uniformly dried product.

In a system like that of the instant invention where the recycled stack gases are used as the source of secondary air to mix with and cool the primary combustion products from the furnace, means must be provided for regulating the latter. In the particular form shown herein, recirculating fan dampers 20 and 48 control the volume of recycled gas returned to the furnace in response to a pressure-sensing element 58 housed inside the combustion zone of the furnace near the point at which the recycled gases enter the latter. In the particular form shown, automatic damper drives 60 and 62 connected to dampers 20 and 48, respectively, operate the latter in response to signals fed thereto from the control mechanism 54 which acts in accordance with information fed thereto by the temperature and pressure sensors. The system can, of course, be controlled manually based upon this same temperature and pressure information, and no particular novelty resides in doing so automatically; therefore, no useful purpose would be served by going into a detailed explanation of the automatic control circuitry, especially when the apparatus necessary to accomplish manual control thereof has already been described and illustrated.

As previously stated, the gas recirculated back through the system will ordinarily have a temperature between approximately 240 F. and 290 F. but, in all cases, its temperature must be above its dew point to eliminate unwanted condensation in the ductwork. The dew point temperature, of course, varies with the pressure and degree of saturation of the exhaust gases, a fact well-known to any competent engineer and one, therefore, that is readily determined under existing operating conditions.

Finally, a few words about the ductwork shown in the accompanying drawing. The illustrated dryer installation including two cyclone separators is, of course, intended as beingmerely representative of many different arrangements that can be used including only one or several such separators, the stack gases from which are combined and returned to the combustion zone. Alternatively, there is no need for introducing the recirculated gases through a single return duct 46. In fact, a better balanced system would certainly result if the flow of recycled gases was split and introduced into opposite sides of the furnace, the return duct on the back side being hidden by the one shown, but being functionally identical thereto.

Having thus described the several useful and novel features of the instant method and apparatus for recycling dryer stack gases, it will be apparent that the several worthwhile objectives for which the device was developed have been achieved. Although but a single embodiment of the invention has been illustrated and described, we realize that certain changes and modifications therein may well occur to those skilled in the art within the broad teaching herein; hence, it is our intention that the scope of protection afforded hereby shall be limited only insofar as said limitations are expressly set forth in the appended claims.

What is claimed is:

1. In combination in a pulp-drying plant: of the type having a gas-fired furnace connected to deliver the hot gaseous products of combustion generated therein to the intake end of a drum dryer rotating about its longitudinal axis, means for delivering wet pulp to the intake end of the dryer drum, means comprising a cyclone separator for centrifugally separating the major portion of the solid constituents into a recovery vessel at the bottom thereof while exhausting the gaseous constituents along with the solids remaining suspended therein through a stack at the top, a pneumatic conveyor means including a dampercontrolled duct containing an induced draft fan connected to receive the gaseous products along with the solids entrained therein from the discharge end of the dryer drum and deliver same tangentially to the cyclone separator, and means for recycling the stack gases containing the major portion of residual solids and removing the latter therefrom, said means including duct means having the inlet end thereof connected tangentially into the cyclone stack in position to draw off that portion of the exhaust gases spiralling upwardly therein containing the major portion of the residual solids and its outlet connected to return the latter to the furnace at a point downstream of its combustion zone where the hot gaseous products of combustion will incinerate the solids and be cooled by the recycled stack gases preparatory to contacting the wet pulp at the intake end of the dryer drum, induced draft fan means mounted in the duct means operative to draw the stack gases and entrained residual solids from the cyclone stack and deliver same to the furnace, damper means mounted within the duct means operative upon actuation to control the flow of recycled gas, and control means responsive to the temperature of the moisture-laden gases leaving the dryer drum operative to actuate the damper-controlled duct so as to regulate the gas flow to the cyclone separator.

2. In a process for drying wet pulp of the type wherein the hot dry gaseous products of primary combustion generated within the combustion zone of gas-fired furnace are first cooled and passed in heat-exchange relation to wet pulp being tumbled in a rotary drum dryer, then sucked from the discharge end of the dryer drum and delivered tangentially by an induced draft fan to a cyclone separator where the major portion of any suspended solids are separated therefrom centrifugally and collected and finally discharged by circulating same up the cyclone stack, the improved method for removing any residual solids left in the exhaust gases while using the latter to cool the hot gaseous products of primary combustion which comprises: drawing off the exhaust gases tangentially from the cyclone stack so as to divert that portion thereof containing most of the residual solids suspended therein and delivering same to the furnace downstream of the combustion zone, said exhaust gases being allowed to cool to a temperature no less than the dew point thereof before reentering the furnace, said exhaust gases being mixed with the hot dry gaseous product of primary combustion to cool the latter and form a substantially inert oxygen-lean mixture therewith that will not support combustion in the dryer drum, and said suspended residual solids being incinerated by said hot gaseous products of primary combustion.

3. The method as set forth in claim 2 in which: the exhaust gases are cooled to a temperature between approximately 240 F. and 290 F. but no lower than the dew point thereof before reentering the furnace.

4. The method as set forth in claim 2 in which: volume and temperature of the exhaust gases are controlled in relation to the volume and temperature of the hot gaseous products of primary combustion such that the mixture thereof enters the dryer drum at a temperature of approximately 1200 F.

5. The method as set forth in claim 4 in which: the exhaust gases are allowed to cool to a temperature no less than the dew point thereof before reentering the furnace.

6. The method as set forth in claim 4 in which: the exhaust gases are cooled to a temperature between approxi- 8 mately 240 F. and 290 F. but no lower than the dew point thereof before reentering the furnace.

References Cited UNITED STATES PATENTS 2,129,673 9/1938 Burns 34-131 X 2,143,505 1/1939 Arnold 34-28 2,715,283 8/1955 Halldorsson 34-79 10 EDWARD J. MICHAEL, Primary Examiner US. Cl. X.R. 34--79, 131 

